Conventional processes for the extraction of zinc mostly involve crushing, grinding and concentration of the ore and then roasting of the resulting concentrate followed by leaching, purification and electrowinning of the zinc. Metals such as copper, nickel and gold have been extracted using heap leaching. See for example U.S. Pat. Nos. 6,168,766; 6,110,253; 4,017,309; 5,196,052; and 4,721,526 where several copper and gold extraction processes are described. Heap leaching of nickel is described in Canadian Patent Application No. 2,155,050. Typically the heap leaching is applied to low grade ores, e.g. <5 g/t for gold and <1% for copper. Therefore, the part of the heap representing the metal being extracted is relatively small compared to the total amount of material in the heap. Nevertheless, the value of the metal being extracted renders the application of heap leaching for these metals economically feasible. Since small amounts of material are leached from such heaps, problems related to decrepitation, slumping and compaction are relatively minor concerns.
Zinc has a lower market value than copper or gold and particularly where the higher grade zinc ores are concerned, the tried and tested methods of concentrate roasting, leaching and electrowinning have been employed for the extraction of the ore. To the applicant's knowledge there are no commercial heap bioleach processes in operation for the extraction of zinc. Therefore, despite the fact that heap leaching has changed the economics in so far as the recovery of gold and copper is concerned, this has not been applied to the recovery of zinc.
A reason for this may be the expectation that leaching of zinc presents problems unique to the heap leaching of zinc ores, such as the precipitation of iron oxides within the heap. Australian Patent Application No. 654322 states that a particular problem of treatment of transition ores in-situ or in heap is the tendency for iron present in the ore or in the treating liquor to precipitate as an insoluble precipitate leading to percolation problems.
While problems relating to decrepitation, slumping and compaction are relatively minor concerns in the heap leaching of copper and gold, these problems threaten to become major concerns with zinc ore leaching where considerable physical changes of the ore, especially with run-of-mine ores of good grade, can be encountered. These changes might be expected to result in percolation and irrigation problems, such as flooding, channelling and cold spots. In addition, permeability problems might be expected in view of the larger amounts of material that need to be leached from the heap in order to render the process economically feasible.
A bioleaching process for the recovery of zinc is described in WO01/18266 but this process is carried out in a reactor tank or vessel and employs oxygen enrichment in order to render the process feasible.
U.S. Pat. No. 6,096,113 describes a tank/heap biooxidation process for recovering a metal from a refractory sulphide ore by splitting the ore in two portions. The first portion is partially biodigested in a reactor to acclimatize the sulphide-digestion microorganism. The partially digested ore is then combined with the second portion. The resulting material is dewatered, biooxidized and subjected to lixiviation.
U.S. Pat. No. 5,429,659 describes a process for recovering precious or base metals from particulate refractory sulphide material comprising contacting the material with an aqueous solution containing a thermotolerant bacteria culture.
Hearne et al (1) propose a process for the recovery of zinc from its sulphide ores or concentrates by an entirely hydrometallurgical route. It consists of bacteria-assisted heap-leaching of sphalerite ore, or leaching zinc concentrate at elevated temperatures with ferric sulphate and re-oxidising the formed ferrous iron with the aid of bacteria in a ferric ion generator. The results of column leaching of different sized ores are reported. The economic feasibility of a moderate-scale heap leach operation is assessed but the authors conclude that for the purely hydrometallurgical recovery route for zinc, industrial acceptance is still some time away until the technology is fully developed and demonstrated on a large scale. The authors further conclude that heap leaching may be beneficial to recover zinc from marginal ore and foresee a process development stage in which a small heap leach/solvent extraction/electrowinning plant is incorporated as an “add on” to another process, i.e. there is no teaching of heap leaching being operated as a stand alone process. The authors also state that zinc solvent extraction, crucial to both ore and concentrate leaching, is not yet satisfactorily solved.
Konishi et al (2) have reported on the kinetics of the bioleaching of ZnS concentrate by Thiobacillus ferrooxidans in a well-mixed batch reactor. Experimental studies were done at 30° C. and pH 2.2 on adsorption of the bacteria to the mineral, ferric iron leaching and bacterial leaching. A mathematical model for bioleaching is presented for quantitatively examining the effects of certain operating variables with the object of selecting optimum bioleaching conditions for zinc concentrates.
Sandström et al (3) have performed bioleaching with moderate thermophilic bacteria at 45° C. and with extreme thermophilic archaea at 60° C. on a complex zinc sulphide ore. The ore was fine grained and contained refractory gold as an additional value. The bioleaching was carried out in continuous stirred tank reactors and although the authors conclude that biooxidation of a complex zinc sulphide ore at 45° C. and 65° C. has proven to be a viable process, especially at the higher temperature, the ore must be finely ground (20 to 28 microns) in order to obtain high zinc recoveries at modest pyrite oxidation.
U.S. Pat. No. 4,401,531 describes a process for the recovery of zinc from secondary zinc raw materials by leaching followed by solvent extraction and electrowinning. However, again the leaching is carried out in stirred tank reactors and in this case no bio-leaching is involved.
Steemson et al (4) describe a process for zinc metal production from zinc concentrates by integrating zinc bioleaching with zinc solvent extraction and electrowinning. The bioleaching again was carried out in reactor tanks. The temperature was controlled at 40° C. to 45° C. In carrying out the process, the concentrate was slurried in water to produce a 6.5% w/w slurry which is then fed to the reactor tanks. See Australian Patent No. 673929 where the Steemson et al process is more fully described.
Krafft et al (5) report on the leaching of two Swedish zinc sulphide ores in columns. Five grain sizes ranging from 4 mm to 128 mm were used. The authors state that, despite intermittent sulphuric acid additions, pH values varied between 2.5 and 3.5 most of the time and it was impossible to avoid precipitation of iron compounds in the columns with the result that for the two smallest grain size fractions of the one ore, the column was clogged by precipitates of iron to such an extent that the leachate could not penetrate the ore mass.
Dutrizac (6) reports on ferric sulphate perolation leaching of a pyritic Zn—Pb—Cu ore and states that zinc recovery from acidic iron-bearing solutions is difficult and that much work still needs to be done in this regard. It is further stated that the problem is especially severe for low zinc concentrations. Problems were also encountered when attempts were made to use higher iron concentrations since part of the iron precipitated.
In light of the above it can be seen that heap bioleaching of ore on its own, or in combination with solvent extraction and electrowinning, has not been established or proven as a viable process for the extraction of zinc on a commercial scale vis-à-vis the conventional processes involving roasting of the concentrate or the bioleaching of finely-ground zinc concentrates in reaction tanks where temperature, pH and bacteria concentration would be expected to be more even than with heap leaching.
It is an object of the present invention to provide a process for the extraction of zinc from an ore by means of heap bioleaching the ore and also to provide an integrated zinc extraction process which includes solvent extraction to produce a pure concentrated zinc solution for the production of zinc metal by electrowinning or for the production of zinc compounds. The invention is the basis of the Cominco HydroZinc™ process.